Influenza vaccines, also known as flu shots or flu jabs, are vaccines that protect against infection by influenza virus. A new version of the vaccine is developed twice a year as the influenza virus rapidly changes. Their effectiveness varies from year to year. The effectiveness in those under two years old and over 65 years old is unknown due to the low quality of the research. There is a high-dose flu vaccine specifically formulated to provide a stronger immune response.Available evidence indicates that vaccinating the elderly with the high-dose vaccine leads to a stronger immune response against influenza than the regular-dose vaccine. (source)